The present invention relates to automatic frequency control (afc) in receiving systems particularly single sideband (SSB) receiving systems. Such receiving systems may be used as radio receivers in vehicles such as land-mobiles, boats and aircraft.
SSB radio is used for economising on bandwidth and transmitter power.
A significant problem to be overcome is to ensure that the local oscillator frequency is such that after it has been mixed with the incoming signal, the correct intermediate frequency (i.f.) is obtained.
One technique would be to use high precision and substantially drift-free crystal controlled oscillators. However such precision oscillators would be so expensive as to render them uneconomical for use in commercial equipment. Further, the higher the frequency, the greater the drift in actual cycles/second (Hz), thus a drift of 1 part in 10.sup.6 is equivalent to 10 Hz at 10 MHz, 100 Hz at 100 MHz and 500 Hz at 500 MHz. Thus the higher the carrier frequency the greater the effect of drift on the demodulated signal. Accordingly some form of automatic frequency control (afc) system is necessary particularly with v.h.f. and u.h.f. equipment for use in vehicles, to take into account not only frequency drift in oscillators but also frequency shift which may occur in the transmitted signal.
SSB techniques may also be used in cable wire or optical signal transmission systems and the techniques herein described could also be applied to these systems.
One automatic frequency control system is disclosed in "Proceedings of the IRE" December 1956 Vol. 44 pages 1854-1873 "Single-Sideband Techniques in U.H.F. Long-Range Communications" by W. E. Morrow, Jr; C. L. Mack, Jr; B. E. Nichols and J. Leonard. This a.f.c. system requires that a 16 MHz pilot carrier, which frequency is also that of a suppressed subcarrier, be transmitted with the modulating signal, both signals being mixed with a u.h.f. carrier. The level of the pilot carrier is 10 db below the peak level of the modulating signal to save transmitter power. In a receiver the pilot carrier is used to provide an afc voltage for a local oscillator, a reference carrier for demodulating the signal, and a voltage for automatic gain control.
In order to provide the afc voltage, the output of an intermediate frequency (i.f.) amplifier is supplied to a first pilot filter the output of which is connected to a second pilot filter and to one input of a phase discriminator. The output of the second filter is connected to a 16 MHz demodulator and to a second input of the phase discriminator. The pilot filters and the phase discriminator produce an afc signal that corrects the local oscillator frequency to keep the pilot carrier centered within the filters. A shortcoming of this known circuit is that the receiver tuning tolerance is typically.+-.200 Hz and is dependent on the filter used to extract the carrier. Accordingly if due to transmitter drift or frequency shift the pilot carrier moves outside the filter range of .+-.200 Hz, the afc circuit may not pull the receiver into the center of the filter and may even lock on to the modulating signal, for example, speech, causing the receiver to be detuned. The problem could be eliminated if the receiver tuning tolerance could be increased.
Another automatic frequency control system for SSB radio is disclosed in "The Radio and Electronic Engineer" Vol. 46 No. 2 pages 69-75, February 1976. In the system described, the carrier is suppressed at the transmitter and no pilot carrier is transmitted, only the modulating signal. A carrier having the same or substantially the same frequency as the transmitter carrier has to be generated in the receiver so that demodulation can take place. In order to be able to generate a carrier of the desired frequency in the receiver it is necessary to be able to detect and correct for any frequency shift which may have occurred during transmission. The SSB system described is based on the detection of inaudible distortion introduced into the modulating signal at the transmitter. The distortion comprises introducing a narrow stop band into the middle of the speech spectrum. In the receiver two band-pass filters are provided. The filters have a narrow pass-band and are centered on either side of the stop band. The outputs of the filters are rectified and combined to produce an automatic frequency control (afc) signal which is used to control the frequency of a local oscillator used in the generation of the carrier frequency. In operation if no frequency shift of the transmitted signal has taken place then the filter outputs are equal and the local oscillator output remains unchanged. If frequency shift does occur within the frequency band bounded by the center frequencies of the filters, an afc signal of appropriate amplitude and polarity related to the amount and direction of the frequency shift from stop-band is produced and is used to adjust the local oscillator frequency accordingly. A limitation of this known SSB system is the range of frequency shift that is able to cope with successfully. On the one hand the stop band introduced in the transmitted signal cannot be too wide because it may impair seriously the modulating signal, which may be speech, and on the other hand if the center frequencies of the filters are spaced too far apart then their outputs would be affected adversely by their passing some of the modulating signal which distorts the afc signal and thereby the frequency of the local oscillator. Hence a compromise has been made in the known system and a lock-in range of .+-.400 Hz is considered practicable. This lock-in range is considered too narrow for satisfactory operation of land-mobiles in built-up and forest areas where the frequency shift may be greater. Furthermore this known system which is complicated and has so far not been entirely satisfactory in practice, has not been evaluated at V.H.F. and U.H.F. carrier frequencies.
The local oscillators in SSB receivers are usually crystal controlled oscillators. Commercially available crystals at economic prices typically produce a drift of .+-.5 parts in 10.sup.6. Such a drift is equivalent to .+-.500 Hz at 100 MHz. Amateur radio equipment is usually equipped with a manually controlled fine tuner, known as a clarifier, which is adjusted to pull-in a received signal and thereby overcome the problems of frequency shift and frequency drift. However the provision of clarifiers in equipment for use in vehicles, for example, land-mobiles, boats and aircraft, is unacceptable because it would distract the operator, particularly if it is a one-man operated vehicle.
Accordingly it is desired to overcome the shortcomings of the prior art equipment and improve the frequency tolerance of SSB receiving system particularly when operated at v.h.f. and u.h.f. frequencies as well as at h.f. frequencies.